The present invention relates to a porous ceramic radiation plate which comprises natural clay as its main component, and particularly to a ceramic composition which comprises at least two kind of organic compounds providing porosity to produce a porous ceramic radiation plate which has a low expansion coefficient and a high thermal radiation transfer coefficient.
Generally, a radiation plate is used in a gas-fired infrared burner (or Schwenk burner) for transferring thermal energy by radiation for baking foods such as biscuits or fish, drying ceramic products or coating metallic varnish, etc. In a gas-fired infrared burner, as shown in FIG. 1, liquified petroleum gas or natural gas is introduced through a gas jet (1) to the combustion channel (2) of the ceramic plate, then, the heat produced from the combustion of the gas is transferred to the pores in the radiation plate (3). Subsequently, the continuously formed triangular projection surface (4) of the plate emits infra-red radiation to heat a certain object such as food or workpiece.
In accordance with the gas-fired infrared burner, the efficiency of the radiation transfer is greatly affected by the porosity of the ceramic radiation plate. For the same porosity, as the radioactivity of small pores is more diffusive but less directional than large pores, the radiation plate which contains a larger number of smaller pores will result in more uniform transfer rate of radiation energy than that which contains larger pores. Moreover, if the same material is used, the radiation plate that has the most pores that are smaller in size will have a larger radiation area, so the efficiency of radiation transfer is increased, and the temperature of the surface of the plate is more uniform. As a result, the radiation plate will not easily crack and thereby have a longer useful life. Conversely, if most of the pores in plate are larger in size, the thermal stress in plate will be large and that will cause cracking on the surface of the plate. In addition, for radiation plates of same porosity, the total numbers of pores in the one containing fewer larger pores will result in greater heat transfer coefficient which will narrow the range of operating control conditions due to the higher possibility of a backfire occurring. As a result, a ceramic plate with small pores at high porosity is preferred to be used in an infra-red burner.
The common materials used in a conventional ceramic plate are metallic oxide or ceramic fibers. However, these materials are quite expensive. Furthermore, the addition of other components to the ceramic composition, such as sawdust, small foamed polystyrene ball, or graphite etc., may cause a problem of producing pores with a larger size. Therefore, there is a decrease in the efficiency of the heat transfer of radiation energy from the surface of the ceramic plate to the objects. On the other hand, it is observed that the time taken for sintering the ceramic composition is too long due to the assurance of product quality, and also preventing the cracking problem. Therefore, the conventional process is difficult to industrialize if both the production rate and the quality of the ceramic plate are required to be high. In conclusion, the efficiency of the radiation rate of the plate is greatly related to the porosity and to the size of the pores.
Conventionally, a porous ceramic composition comprises porous ceramic materials as its basic component, as well as some other additives such as inflammable compounds, foaming agent, or binding agent to form porosity. For example, a composition of porous ceramic material, such as diatomaceous earth etc., mixed with clay is sintered to produce a porous ceramic product. Another example, porous ceramic product can be obtained by sintering a composition of ceramic particles (e.g., chamotte, SiO.sub.2, and Al.sub.2 O.sub.3) which sizes are within a certain range, mixed with a suitable coal solvent.
For producing porous insulating or refractory material, usually, an organic component is added to the material, e.g., sawdust, graphite, coal powder, or small foamed polystyrene ball. However, it is known that several disadvantages are observed during the sintering process.
In accordance with the addition of sawdust to the porous refractory composition, the size of the sawdust is suggested to be less than 200 mesh. If the material contains some sawdust with a larger size, the product will easily crack, since the elasticity of the wood will cause a stress in the product. Furthermore, if sawdust is the only organic component mixed with the material to produce porosity, the volatile or inflammable component will be released at a certain narrow range of temperature during the sintering process, as a result, the quality of the product is low due to its cracking problem. Therefore, the sintering furnace should be turned off at 400.degree.-500.degree. C., and the sawdust should be slowly burnt off in the closed furnace. After all the sawdust has been burnt off, the furnace is turned on again and heated to sintering temperature to complete the sintering process. Therefore, the sintering time is too long whereby a production rate can not attained that is up to standards.
For the ceramic composition with added small foamed polystyrene balls, the distribution of the sizes of pores in the ceramic product is not homogeneous, i.e., it contains smaller pores as well as larger pores. Therefore, the efficiency of radiation transfer is greatly decreased. Also the time required for sintering the ceramic product is quite long due to the need for preventing cracking.
As the ignition point of graphite is too high that is can not easily be burnt off, the ceramic product doped with graphite will take a long time to complete its sintering process and much fuel is required to be used up that production cost is raised. For doping with coal powder, the quality of the ceramic product is low because of a cracking problem, since some coal powder may remain in the product.
According to the patents related to the process for making ceramic radiation plates, Japan Laid Open Patent No. 56-56514 discloses a ceramic composition which contains Al.sub.2 O.sub.3, SiO.sub.2, and SiC. For Japan Laid Open Patent No. 142915, Al.sub.2 O.sub.3, SiO.sub.2, MgO, ZrO.sub.2, TiO.sub.2, cordierite and mullite are used as the main components in a ceramic composition. U.S. Pat. No. 4,504,218 uses ceramic fiber Al.sub.2 O.sub.3 and SiO.sub.2 as the main components, while doping with Li.sub.2 O, the oxides of Ni, Co, Mn, Fe, Cr, V, and Ti, as well as fireclay. However, the material used in the above-mentioned patents are fine ceramic fiber or powder that causes the production cost to be high.